Positionable popups?

Can one of the great knowlegeable people out there tell me how to make positionable pop ups, such that the pop up goes to the top right/left middle etc. I can make the window go to a pixel co-ordinate, but not a universal pos such as 100% top right, as it depends on the res size of the screen.

If possible a pasted snippet of a sample would be great
Cheers

[This message has been edited by Bruce Anderson (edited 11-16-99).]